addtocart - tuning

modified how the add to cart in a prodlist works. The addToCartButton
had the action for showing the cart in a modal hard coded. Moved that to
the form so that other actions could easily be defined.
new cart extension 'quickadd' for add to cart

An alternative action for cart display after adding an item to the cart.

Instead of the full-blown cart modal, this will animate a small element
in from the right and display either the item added (if successful) or
the error message.

Also will auto-retract after five seconds and includes a shop more
button (which also retracts the element) and a checkout button.